1. Field of the Invention
The present invention relates to an optical molding material, and an optical disk substrate and a production method thereof, and more particularly to an optical molding material which is high in transfer accuracy and toughness, small in birefringence, particularly, birefringence occurred in a thicknesswise direction, good in optical disk properties and free of occurrence of silver streak, or to an optical molding material which is small in the variation range of retardation depending on light wavelength and good in optical properties when it is molded into an optical disk substrate by injection molding in particular, and also to an optical disk substrate making use of such a molding material, and further to a production method of an optical disk substrate which is free of occurrence of silver streak upon its production, high in transfer accuracy and toughness, small in warpage, narrow in scatter of birefringence properties, particularly, concyclic birefringence, and good in optical disk properties.
2. Description of the Background Art
In recent years, amorphous plastics including polycarbonate and polymethyl methacrylate have been utilized and applied in a field of optical disks such as compact disk (CD), CD-ROM, DVD and MO and optical materials for optical lenses, optical fibers, light guide plates, etc., and their various new uses have been investigated.
However, polycarbonate and the like is high in dependence of retardation (birefringence) on wavelength, and their molded products have small retardation at a specific wavelength, but become great in retardation in the case where light having any other wavelength is used. Therefore, such properties have been unsatisfactory as product characteristics required.
More specifically, optical instruments have currently taken striking strides. For example, drives capable of conducting recording and reproduction on plural optical disks (for example, 780 nm in the CD system and 650 nm in DVD system) different in wavelength of light used are going to come on the market. Further, in the next generation drives, the development of those capable of coping with blue laser of short wavelength (about 400 nm) are being advanced. Therefore, there is a demand in the market for development of optical molding materials stable in optical properties (retardation) over a wide wavelength range.
In the case of optical disks that information is read and written by using a fine laser beam, their product characteristics are greatly affected by differences in their optical properties. Therefore, when a great change is caused in retardation depending on wavelength used, production conditions corresponding to individual media must be newly preset. Accordingly, if a material exhibiting comparatively fixed retardation over a wide wavelength range is provided, great merits such as improvement in production efficiency would be obtained. Therefore, there is an increasing demand for development of such a material. In addition, an optical disk capable of reading and writing by a plurality of laser beams can also be developed, if retardation from a low wavelength to a high wavelength becomes stable. Therefore, they may become a material and optical disk for developing new technical fields.
On the other hand, cyclic polyolefin resins have such features that they have a high glass transition temperature owing to the stiffness of their main chain structure, are amorphous and high in light transmittance because a bulk group is present in the main chain structure, and exhibit low birefringence properties because they are small in anisotropy of polarizability, and attract attention as transparent thermoplastic resins excellent in heat resistance, transparency and optical properties.
Such cyclic polyolefin resins are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 132625/1989, 132626/1989, 218726/1988, 133413/1990, 120816/1986 and 115912/1986.
In recent years, the cyclic polyolefin resins have been investigated in application thereof to fields such as various kinds of optical materials, and sealing materials such as sealants for optical semiconductors.
However, the cyclic polyolefin resins are comparatively low in toughness due to their bulky structure and hence cause burr by gate cutting and circumferential burr by handling and the like upon production of products such as optical disks and optical lenses, so that yield of such products may be lowered, or secondary processability thereof may be markedly inhibited.
On the other hand, in order to improve the toughness, it is considered to increase the molecular weight of a cyclic polyolefin resin. According to such a method, the toughness is improved, but the birefringence of molded products obtained from such a resin may become great to the contrary, so that the optical properties may be deteriorated to a marked extent.
It is generally conducted to control the molecular weight distribution for the purpose of balancing the toughness with the birefringence. In such a case, however, it has been relatively difficult to balance the toughness with the birefringence, and particularly, it has been difficult to obtain those small in birefringence in a thicknesswise direction or narrow in scatter of birefringence at concyclic positions (on a circumference of the same radius position), and further it has been considerably difficult to obtain a molded product having good optical disk properties with high yield because silver streak occurs on the surface of the molded product upon its production by injection molding or the like. In particular, a thin-wall molded product having a great surface area such as an optical disk is easy to cause silver streak, and it is difficult to reduce birefringence occurred upon the molding. In addition, since burr is easy to be formed upon gate cutting or handling, subsequent secondary processing for forming a metal film, organic film and/or the like involves a problem that the yield of the product is lowered.
In such an application field of optical disks, recording density of information has become high with the change of times, and so more improved optical properties have been required. Among these, there is great nee d to enlarge the numerical aperture of an objective lens due to the reduced diameter of a laser light spot used in reading and writing of information to an optical disk, and so great importance is attached to the influence of birefringence properties, particularly, birefringence in a thicknesswise direction or scatter of birefringence at concyclic positions on optical disks.
The present invention has been made on the basis of the foregoing circumstances.
It is the first object of the present invention to provide a molding material suitable for optical uses, or an optical molding material which is high in transfer accuracy and toughness, small in birefringence, particularly, birefringence occurred in a thicknesswise direction, good in optical disk properties and free of occurrence of silver streak when it is molded into an optical disk substrate by injection molding in particular, and an optical disk substrate made of the molding material.
The second object of the present invention is to provide a molding material suitable for optical uses, or an optical molding material which is high in transfer accuracy, narrow in concyclic scatter of retardation, small in the variation range of retardation in a wavelength range of from 400 nm to 830 nm in particular and good in optical properties in the above-described wide wavelength range when it is molded into an optical disk substrate by injection molding in particular, and an optical disk substrate made of the molding material.
The third object of the present invention is to provide a production method of an optical recording medium which is free of occurrence of silver streak upon its production, high in transfer accuracy and toughness, small in warpage, narrow in scatter of birefringence properties, particularly, concyclic birefringence, and good in optical disk properties.
According to the present invention, there is thus provided an optical molding material comprising a cyclic polyolefin resin having a glass transition temperature (Tg) not lower than 120xc2x0 C., but not higher than 170xc2x0 C., wherein the melt viscosity of the cyclic polyolefin resin at 260xc2x0 C. is 5,000 to 50,000 poises as measured at a shear rate(xcex3) of 10 secxe2x88x921 and 200 to 1,000 poises as measured at a shear rate(xcex3) of 10,000 secxe2x88x921, and the melt viscosity of the cyclic polyolefin resin at 320xc2x0 C. is 500 to 5,000 poises as measured at a shear rate(xcex3) of 10 secxe2x88x921 and 100 to 800 poises as measured at a shear rate(xcex3) of 10,000 secxe2x88x921.
According to the present invention, there is also provided an optical disk substrate obtained by molding the optical molding material described above, wherein the recording area portion of the optical disk substrate satisfies the following relationships:
|Nxxe2x88x92Ny|xe2x89xa60.0001;
|Nxxe2x88x92Nz|xe2x89xa60.002; and
|Nyxe2x88x92Nz|xe2x89xa60.002;
wherein Nx is a refractive index in a radial direction in the plane of the disk, Ny is a refractive index in a direction perpendicular to the radial direction in the same plane, and Nz is a refractive index in a direction perpendicular to the same plane.
According to the present invention, there is further provided an optical molding material comprising a cyclic polyolefin resin, wherein a variation range of retardation of the resin is within xc2x110% of the retardation at a wavelength of 590 nm when the wavelength of light is changed from 400 nm to 830 nm.
According to the present invention, there is still further provided an optical molded product obtained by molding a cyclic polyolefin resin, wherein a variation range of retardation of the molded product is within xc2x110% of the retardation at a wavelength of 590 nm when the wavelength of light is changed from 400 nm to 830 nm.
According to the present invention, there is yet still further provided an optical disk substrate obtained by molding a cyclic polyolefin resin, wherein a variation range of retardation of the optical disk substrate is within xc2x110% of the retardation at a wavelength of 590 nm when the wavelength of light is changed from 400 nm to 830 nm.
According to the present invention, there is yet still further provided a method for producing an optical disk substrate, which comprises injection-molding a thermoplastic resin comprising a cyclic polyolefin resin under conditions of a resin temperature of (Tg+100)xc2x0 C. to 400xc2x0 C. (Tg being a glass transition temperature of the thermoplastic resin; the same shall apply hereinafter), a mold temperature of (Tgxe2x88x92100)xc2x0 C. to (Tgxe2x88x925)xc2x0 C., a shear rate of 4xc3x97104 to 1xc3x97106 secxe2x88x921 and average residence time of the thermoplastic resin in a molding machine of 5 to 200 seconds.
In the above, the cyclic polyolefin resin may preferably be a specific cyclic polyolefin resin which is at least one selected from the group consisting of a ring-opening polymer of a monomer (hereinafter referred to as xe2x80x9cspecified monomerxe2x80x9d) represented by the general formula (I), which will be described subsequently, a ring-opening copolymer of the specified monomer and a copolymerizable monomer, hydrogenated polymers of these ring-opening polymer and ring-opening copolymer, and a saturated copolymer of the specified monomer and an unsaturated double bond-containing compound.
The optical molding materials according to the present invention are molding materials suitable for optical uses, are high in transfer accuracy and toughness, small in birefringence, particularly, birefringence occurred in a thicknesswise direction, good in optical disk properties and free of occurrence of silver streak when they are molded into optical disk substrates by injection molding in particular, and can provide product having such good properties at a high yield.
Therefore, the invention can contribute the enlargement of applications of the cyclic polyolefin resins in the field of optical materials, particularly, optical disks, which is of extremely high industrial value.
The optical molding materials according to the present invention are molding materials suitable for optical uses stable in optical properties (retardation), are high in transfer accuracy, narrow in concyclic scatter of retardation, small in variation range of retardation in a wavelength range of from 400 nm to 830 nm, i.e., at most xc2x110% of retardation at a wave length of 590 mm, and good in optical properties in the wide wavelength range, exhibit excellent optical disk properties, and can provide products having such good properties when they are molded into optical disk substrates by injection molding in particular. Therefore, when optical disk substrates different in wavelength used are molded, time and labor required for newly presetting production conditions corresponding to individual media are saved since the variation range of retardation due to change of wavelength is small, thereby achieving great force saving, such as more improved production efficiency. Further, the retardation from a low wavelength to a high wave length is stabilized, and so optical media capable of reading and writing by a plurality of lasers may also be developed, which is of extremely high industrial value in that they may be materials and optical disks for developing techniques in the next generation.
The production method of an optical recording medium according to the present invention provides an optical recording medium which is free of occurrence of silver streak upon its production, high in transfer accuracy and toughness, small in warpage, narrow in scatter of birefringence properties, particularly, concyclic birefringence, and good in optical disk properties. By this method, the invention can contribute the enlargement of applications of the cyclic polyolefin resins in the field of optical disks, and moreover can provide high-quality optical recording media, which is of extremely high industrial value.